The present invention relates to an add-on type secure communication control system that can be adapted readily, without any modification, to most conventional communication devices, such as facsimiles, used for transmission of images over a public switched network.
A great number of information/data encoding and decoding systems, have been proposed. One example of a facsimile with a security function is disclosed in Japanese Patent Unexamined Publication No. 1984-221167. This facsimile is designed to have a built-in security function or to be connected to an external security function via a specific interface. It also uses a non-standard function (NSF) signal specified as an optional function of CCITT's recommended communication protocol for automatic identification of the security function, and adds security function-related information to the NSF signal.
According to the invention in the above-mentioned Japanese publication, secure communication is limited to a small population of identical data circuit terminating equipment; that is, a station can choose only an identical station with which to communicate.
The facsimile based on the invention in the above-mentioned Japanese publication is designed to have a built-in security function or to use a security function as an external device. As a result, a conventional facsimile cannot use the security function according to the above-mentioned Japanese publication without modification or design change. In contrast, according to the present invention, any conventional communication device can be used directly for security communication without referencing any specific optional signal (e.g., NSF signal).